Such a system that includes a plurality of processing devices connected with one another through a communication medium to exchange information and to realize various types of signal processing has been used in a number of fields.
Especially in the field of vehicles, the control of a vehicle has been undergoing transition from mechanical control to electrical control. Hence, such a system in which a large number of vehicle-mounted processing devices, i.e. ECUs (Electronic Control Units) are mounted and respectively connected to vehicle-mounted communication lines in order to transmit and receive data for control to realize various functions becomes widely used.
In the field of vehicles, while a large number of ECUs are mounted as described above, it is desired to reduce power consumption for the entire vehicle. It is thus required to reduce power consumption of the entire system including ECUs, while a number of techniques for individually reducing power consumption of each ECU have been proposed.
In a structure in which existing in-vehicle ECUs are connected by bus connection to a communication line, the ECUs connected to the same communication line are basically activated all at once. When switching to a power-saving state, all the ECUs that are connected to the same communication line are ready to be switched to the power-saving state. Each of the ECUs is switched to the power-saving state only after communication on the communication line is stopped. Naturally, when a communication signal is sent out from any one of the ECUs to the same communication line, all the ECUs connected thereto sense the input of the communication signal and are activated. However, depending on circumstances, some ECUs may be unnecessary to be activated among the ECUs connected to the same communication line. Even in such a case, when all the ECUs connected to the same communication line are simultaneously activated and switched to the power-saving state, power saving as the entire system is insufficient. In order to reduce power consumption, it is desired to allow each of the ECUs connected to the same communication line to individually be activated or switched to the power-saving state.
A technique, therefore, has been devised in that information regarding activation or switching to the power-saving state of an ECU is separately transmitted and received between ECUs so that each ECU can individually be activated or switched to the power-saving state on the basis of the information. For example, the ECUs are connected through a dedicated signal line so as to separately transmit and receive signals different from an existing communication protocol (e.g., CAN (Controller Area Network)), to mutually control activation or switching to the power-saving state. This, however, incurs a waste such that a dedicated signal line is required in the system. Especially in the field of vehicles, reduction in weight of a vehicle is encouraged for improved fuel consumption. Increase of an excess signal line should thus be avoided.
Japanese Patent Application Laid-Open No. 2008-193606 discloses a system and a method for superposing the second signal on a twist pair cable in the same phase between ECUs communicating with each other according to the CAN protocol using a differential signal, making it possible to transmit data different from a communication signal in CAN.
Moreover, Japanese Patent Application Laid-Open No. 2008-290538 discloses a system for superposing a different modulation signal to communication according to the CAN protocol, to increase an amount of information that can be transmitted and received without the need for an extra communication line.